An Introduction to Glide at Static Pressure
We’ve been pretty spoiled in residential and light commercial in the USA because we haven’t needed to deal with glide much. R-22 has no glide and R-410a is a near-azeotropic blend (which means it has almost no glide).
The days of being able to ignore glide are coming to an end.
Understanding the bubble point, dew point, and mid-point
Carrier has announced their replacement for R-410a will be R-454b, which it will call Puron Advanced, which still has very little glide (only 0.2°F), but many of the other options have a rather severe glide.
Glide comes down to the fact that some blended refrigerants boil and condense over a range of temperatures rather than at a single pressure/temperature point.
The point at which it is fully liquid before subcooling (or the point of the very first bubble in the liquid) we call bubble point. We use the bubble point to calculate subcooling.
The point when the mixture becomes fully vapor before superheating (or the first drop of liquid dew in a vapor) we call the dew point, and we use it for calculating superheat.
Zeotropic blends (blends with glide) have several impacts on the system, but the one we notice most is in the evaporator. When blend with glide enters the evaporator coil, it will start by boiling at a lower temperature, and as it moves through the coil, the refrigerant temperature will increase until it hits the dew point before it starts to superheat. This means that neither the dew nor the bubble temperature is really the evaporator temperature, but the true effective evaporator temperature is somewhere in the middle, which we call the mid-point.
Because some of the refrigerant flashes off right at the start of the evaporator, the effective midpoint isn’t really the middle between the dew and bubble; it tilts more towards the dew. Emerson recommends a more accurate estimate would account for that “inlet quality.” So merely multiply bubble by 0.40, dew by 0.60, and add the two together to get a more accurate evaporator midpoint.
But let’s say you connect to a system that is off or connect gauges to a tank and want to know for sure that that refrigerant you think is in the tank or system is what you think it is: Do you use bubble, dew, or mid-point for static pressure?
You use bubble. Now I’ve not had anyone fully explain why, but it stands to reason in that in the static state the majority of the refrigerant mass in the system (or tank) is in the liquid state. Since it is neither in the process of boiling or condensing, then it would be at the bubble point.